Production of ferritic stainless steels containing zirconium

ABSTRACT

Ferritic stainless steel slabs which can be rolled into strip and sheets free from surface defects are made by incorporating zirconium in the steel in amounts between about seven times and about fourteen times the combined carbon and nitrogen percentages by weight and grinding the slab surface.

United States Patent [191 Aronson et al.

[ Dec. 16, 1975 1 PRODUCTION OF FERRITIC STAINLESS STEELS CONTAININGZIRCONIUM [75] Inventors: Arthur H. Aronson, Pittsburgh;

Charles R. Rarey, Eighty-Four, both of Pa.

[73] Assignee: Jones & Laughlin Steel Corporation,

Pittsburgh, Pa.

[22] Filed: Mar. 17, 1972 [21] Appl. No.: 235,849

[52] US. Cl. 75/130.5; 75/126 C; 75/126 F; 148/135 [51] Int. Cl. C22C33/00 [58] Field of Search 75/130.5, 126 F, 126 C; 148/37, 135, 12

[56] References Cited UNITED STATES PATENTS 1,925,182 9/1933 Feild 75/122,087,431 7/1937 Feild 148/135 2,139,538 12/1938 Becket et a1 148/1352,153,906 4/1939 Aborn et a1 148/135 2,230,531 2/1941 Digby 148/1352,340,461 2/1944 Gage et al..... 148/135 2,597,173 5/1952 Patterson75/130.5 2,624,668 1/1953 Binder ..t 148/135 2,693,414 11/1954 Dunn eta1. 75/130.5 3,455,681 7/1969 Moskowitz et a1 148/37 3,499,802 3/1970Lagneborg 148/37 3,574,602 4/1971 Gondo et a1... 75/126 F 3,700,43210/1972 Brickner 75/126 F Primary Examiner-Walter R. SatterfieldAttorney, Agent, or Firm-T. A. Zalenski [57] ABSTRACT and grinding theslab surface.

3 Claims, No Drawings PRODUCTION OF FERRITIC STAINLESS STEELS CONTAININGZIRCONIUM suitable for rolling into strip free from surface defects.

The stainless steels with which our invention is concerned are thosefree from deliberately added austenite formers and which contain ferriteformers in amounts sufficient to render them ferritic at alltemperatures up to their melting temperatures. Chromium is, of course,

a ferrite former. The unavoidable impurities carbon and nitrogen areaustenite formers. If chromium is the only ferrite former present,steels containing more than about chromium are fully ferritic as long asthe combined percentage content by weight of carbon and 20 nitrogen isunder about 0.20%, which represents commercial tolerances. Steels withlower chromium contents may be fully ferritic if their combined carbonand nitrogen contents are low enough, or if they contain elements suchas columbium which combines with carbon to neutralize its austeniteforming properties. Likewise steels with lower chromium contents may befully ferritic if they contain additional ferrite formers. For example,ordinary AISI Type 430 steel which contains 16 to 18% chromium is notfully ferritic but AISI Type 436 steel which is AISI Type 430 steel withadded columbium and molybdenum, is essentially fully ferritic.Molybdenum is a ferrite former. All these steels also contain up toabout 1% manganese and silicon,

0.040% maximum phosphorus and 0.030% maximum sulfur. Fully ferriticsteels containing less than about 16% chromium are not commerciallypracticable and are not encompassed in our invention. Those steelscontaining from about 23% to about 27% chromium are of particularinterest, and are exemplified in AISI Type 446. The presence of chromiumin excess of about 27% does not appear to result in properties of anycommercial interest.

In the production of both stainless and carbon steels for strip andsheets, it is conventional to teem the melt into molds and to convertthe cast bodies so made into strip or sheets by hot rolling, followednormally by cold rolling. The steel is introduced into the hot rollingmill as slabs, that is, bodies 6 inches to 8 inches or so thick andapproximately the width of the strip or sheet desired. These slabs maybe produced directly by teeming the melt into molds of appropriatedimensions or they may be produced indirectly by teeming the melt intomolds of larger dimensions and reducing those ingots to slab size byforging or rolling in a blooming mill or the like. In either case, thequality of the surface of the strip or sheet finally produced isdirectly dependent on the surface condition of the slab. In order tominimize surface defects in the strip or sheets, it is well-nigh faceand remove defects such as cracks, scabs, laps and the likebefore theslab is rolled. In the production of carbon steels, the defects areusually removed by chipping with air operated chisels or scarfing withoxy-acetylene torches. The defects are cut or melted out so as to leaverelatively broad shallow depressions in the slab surface which leave nomark on the rolled strip. In the production of stainless steels, thesurface defects are generally removed by grinding with a grinding wheel.Frequently, the entire surface of the slab is ground.

A major problem in the production of slabs of fully ferritic stainlesssteel is that of surface conditioning. The steel in slab form is highlysensitive to grinding and tends to crack in the course thereof. Thetendency of the steel to crack during grinding seems to be morepronounced at lower temperatures, which has lead some shops to scheduleproduction of fully ferritic stainless steels only during periods ofmild weather, or to warm the slabs.

It is the principal object of our invention, therefore, to provide aprocess of making surface ground slabs of fully ferritic steel free fromcracks. Other objects of our invention will appear in the descriptionthereof which follows.

We have found that 'slabs of fully ferritic stainless steelentirelysuitable for the production of defect-free strip and sheet can be madeby incorporating in the steel'melt zirconium in amounts between aboutseven times and fourteen times the combined weight percentage of thecarbon and nitrogen of the melt, processing the steel into slabs, andgrinding the slab surface. The addition of zirconium effectivelysuppresses the formation of cracks during surface grinding.

The effectiveness of our process is demonstrated by the results in theaccompanying table. That table includes results of four zirconium-freeheats, and fifteen heats containing zirconium in different amounts, theheats being otherwise of comparable compositions. All heats wereprocessed into slabs of the same size which were surface ground in thesame way. The grinding comprised one traverse of a 4 inch wide slabsample in quarter-inch steps using a 2 inch wide coarse grinding wheelof commercial manufacture (Dayton wheel 8A24J l5V25) with a grindingdepth of 0.010 inch.

Each slab sample after grinding was macroetched, coated with apenetrating dye, and photographed. The extent of cracking wasquantitatively determined by placing over the photograph a parallel linegrid and counting the crack intersections per inch of grid.

The table shows that the crack incidence in the untreated samples wasfar greater than that in the samples containing zirconium in amountsbetween about seven times and about 14 times the combined percentage ofcarbon and nitrogen. In fact, four of the eight heats in this bracketdisplayed no cracks at all. The tabulated data also show that somewherebetween about 12 and about 16 times the combined percentage of carbonand nitrogen, which we take to be about 14 times, the zirconium additionloses its effectiveness as a cracking supuniversal practice in steelmills to inspect the slab sur- 6O presser.

Heat No. %Cr %Mo %C %N %Zr %Zr/(%C %N) Crack Intersections/In. of GridNo Addition 5181A 22.4 1.7 .028 .052 50.8 8182A 23.4 1.8 .020 .031 49.65197A 26.3 .004 .030 74.1

-continued Heat No. %Cr %Mo %C %N %Zr %Zr/(%C %N) CrackIntersections/In. of Grid We claim: weight between about seven times andabout fourteen .below its melting point, zirconium in percentage bytimes the combined carbon and nitrogen percentages by weight toeffectively suppress the formation .of cracks during surface grinding ofa slab formed from the melt, balance iron, processing the melt into aslab, and grinding at least one surface thereof.

2. The process of claim 1 in which the chromium content of the melt isbetween about 23% and 27% by weight.

3. The process of claim 1 in which the melt contains molybdenum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent ,9 6 4Dated December 16, 1975 Inventofls) Arthur H Aronson and Charles R RareyIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Column 3, line 17: "0.26" should be .026

and "0.21" should be -.021

Signed and Scaled this sixteenth D ay 0f March I 9 76 [SEAL] AIICSI.

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner irflatentsand Trademarks

1. THE PROCESS OF MAKING A FULLY FERRITIC STAINLESS STEEL SLABCHARACTERIZED BY ITS ABILITY TO RESIST SURFACE CRACKING DURING GRINDINGAND THUS BE SUITABLE FOR ROLLING INTO STRIP CONSISTING ESSENTIALLY OFPREPARING A MELT OF STEEL CONSISTING ESSENTIALLY OF CARBON, NITROGEN,APPROXIMATELY 1% MAXIMUM MANGANESE AND SILICON, APPROXIMATELY 0.04%MAXIMUM PHOSPHORUS, APPROXIMATELY 0.03% MAXIMUM SULFUR, FERRITE FORMINGELEMENTS INCLUDING A MINIMUM OF APPROXIMATELY 16% CHROMIUM TO RENDER THERESULTING STEEL FERRITIC AT ALL TEMPERATURES BELOW ITS MELTING POINT,ZIRCONIUM IN PERCENTAGE BY WEIGHT BETWEEN ABOUT SEVEN TIMES AND ABOUTFOURTEEN TIMES THE COMBINED CARBON AND NITROGEN PERCENTAGES BY WEIGHT TOEFFECTIVELY SUPRESS THE FORMATION OF CRACKS DURING SURFACE GRINDING OF ASLAB FORMED FROM THE MELT, BALANCE IRON, PROCESSING THE MELT INTO ASLAB, AND GRINDING AT LEAST ONE SURFACE THEREOF.
 2. The process of claim1 in which the chromium content of the melt is between about 23% and 27%by weight.
 3. The process of claim 1 in which the melt containsmolybdenum.